This invention relates to material for a housing of electronic components for protecting the electronic components from external electromagnetic wave noise.
In recent years, digital apparatus, including microcomputers, have been developing remarkably and are used widely in office equipment, household electric appliances, automobiles, etc. In addition, microcomputers with higher integration and higher clock frequency have been increasing in number. However, electromagnetic wave noise generated in the electronic components, or the like, is carried away on signal conductors, such as flat cables, or has a direct effect on the electronic components. Such electromagnetic wave noise brings about breakdowns, troubles, and faulty operations of the electronic components controlled and driven by weak electric current. Moreover, since magnetic recording elements with high density in the electronic components are made of a magnetic substance by small magnetic energy, data recorded in these elements is extinguished by external magnetic fields caused by magnets, or the like.
In the prior art, the material used for a housing of electronic components, for example a conductive metallic material, such as iron or copper, or synthetic resin material containing carbon black particles as a conductive filler, shields electronic components from external electromagnetic wave noise.
However, each of these two kinds of prior-art material is imperfect and has problems.
Synthetic resin material bestowed with conductivity by carbon black contained therein has two advantages: it is light and can be molded into various shapes with more freedom by injection molding, vacuum molding, etc. On the other hand, it has a disadvantage: the more carbon black particles it contains, the less strength the synthetic resin has. Therefore, the content of carbon black particles has an upper limit, because the synthetic resin material must maintain a certain strength so as to house electronic components and resist external force. Therefore, conductivity, or electrical resistivity, of the housing of electronic components made of such conductive synthetic resin can only be reduced to about 100 ohm.cm. In addition, since the conductive synthetic resin material is a non-magnetic substance, magnets must be attached on the surface of the housing. The magnets cannot perfectly shield the electronic components from a low-frequency magnetic field, and breakdowns or faulty operations of the electronic components are brought about. The housing of electronic components is referred to as `housing` below.
Conductive metallic material, the other prior-art material for a housing, is bestowed with high permeability and low electric resistivity of about 10.sup.-5 ohm.cm. Unlike the aforementioned conductive synthetic resin material, a housing made of conductive metallic material, such as iron plate, can concentrate on its surface magnetic flux of electromagnetic wave noise from electronic components and also prevent electronic components from troubles or faulty operation caused by a low-frequency magnetic field. However, the metallic plate must be thick enough to maintain designated strength, so it cannot be lightened to a sufficient extent. Moreover, metallic material is molded into only limited shapes by a press molding, partly because irregular parts orthogonal to the driving direction of a press die are difficult to shape.